This invention relates to the field of unstacking sheets stacked in a feeder, in particular, semi-rigid PVC sheets stacked in a feeder.
In the state of the art, stackers are known to include a drive mechanism with a vacuum cup which is alternatively brought close, put into contact then moved away from the stack of sheets. The vacuum cup is laid against the top sheet of the stack and then carries away the top sheet to bring it away from the stack before laying it down on another work area, for example, on a cut transported by a vacuum conveyor. U.S. Pat. No. 5,254,071 describes an unstacker which operates according to this principle.
Such a device is not totally satisfactory. Vacuum cup kinematics produces an initial separation of a central area of the top sheet, which slightly bulges before complete unstacking. This bulging arises from the sticking phenomenon of superimposed sheets, notably resulting from electrostatic forces formed between two sheets of insulating materials such as PVC. The unstacking is not properly performed and it may cause malfunctions of the unstacker.
Accordingly, it would be advantageous to improve the unstacking of semi-rigid sheets, in particular, PVC sheets stacked in a feeder. In particular, it would be advantageous to avoid the drawback of the sticking of two consecutive sheets causing two sheets to be picked up simultaneously, or causing inaccurate positioning of the sheet consecutive to the top sheet which has just been picked up.
This invention relates to a method of unstacking sheets stacked in a feeder including a) bringing a vacuum cup close to the surface of a top sheet of the stack, b) contacting the vacuum cup with the surface of the stop sheet in an off-center area of the top sheet, and c) bringing the vacuum cup away from the stack after seizing the top sheet, such that the vacuum cup is inclined, at the beginning of step c) to cause the top sheet to be deformed, thereby producing a side separation prior to complete separation of the top sheet.
This invention also relates to an apparatus for unstacking stacked sheets including a rotatable turn table driving at least a support of a vacuum cup and including a vacuum cup support formed by a jointed arm that is movable with respect to a swivel axis extending substantially perpendicularly to a peripheral area of the rotatable turn table, wherein the jointed arm is guided by a guide connecting rod, one end of which is translationally guided by a linear bearing supported by an axis of rotation of the rotatable turn table, and the other end of which is connected to the jointed arm through a swivel.